Re: [PATCH] virtio-gpu: Do not wait for the main thread during reset

[Akihiko Odaki]

On 2026/03/03 21:31, Alex Bennée wrote:
Akihiko Odaki <od...@rsg.ci.i.u-tokyo.ac.jp> writes:
virtio-gpu waits for the main thread to destroy resources and replace
surfaces, but it occasionally results in deadlock, so remove the code
to wait.

In particular, when running a test case[1] the main thread may wait for
the vCPUs to pause during shut down while a vCPU may be concurrently
resetting virtio-gpu.

vCPU actually does not need to perform resource destruction and surface
replacement synchronously, but it only needs to ensure correct ordering
among virtio-gpu operations. virtio-gpu-gl already exploits this fact to
ensure that virglrenderer is reset on the main thread; instead of
synchronously resetting virglrenderer when the device is being reset,
it resets virglrenderer just before processing the first command after
the device reset arrives.

Take advantage of this fact by simply removing synchronization between
the main thread and the resetting vCPU thread. The request to destroy
resources and replace surfaces is scheduled earlier than any virtio-gpu
command that may be queued after resetting ensures correct ordering, so
we do not need to make additional effort to ensure ordering.

[1]
https://lore.kernel.org/qemu-devel/20251014111234.3190346-9-alex.ben...@linaro.org/

This doesn't seem to make any difference for me with the current state
of the tree.

It depends on timing, so perhaps it may not be repdocued on some 
environments.



Fixes: a41e2d97f92b ("virtio-gpu: reset gfx resources in main thread")
Signed-off-by: Akihiko Odaki <od...@rsg.ci.i.u-tokyo.ac.jp>
---
  include/hw/virtio/virtio-gpu.h | 2 --
  hw/display/virtio-gpu.c        | 9 ---------
  2 files changed, 11 deletions(-)

diff --git a/include/hw/virtio/virtio-gpu.h b/include/hw/virtio/virtio-gpu.h
index 9f16f89a36d2..77d2214238df 100644
--- a/include/hw/virtio/virtio-gpu.h
+++ b/include/hw/virtio/virtio-gpu.h
@@ -193,8 +193,6 @@ struct VirtIOGPU {
      QEMUBH *ctrl_bh;
      QEMUBH *cursor_bh;
      QEMUBH *reset_bh;
-    QemuCond reset_cond;
-    bool reset_finished;
  
      QTAILQ_HEAD(, virtio_gpu_simple_resource) reslist;
      QTAILQ_HEAD(, virtio_gpu_ctrl_command) cmdq;
diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c
index 3a555125be60..1294e1d482ed 100644
--- a/hw/display/virtio-gpu.c
+++ b/hw/display/virtio-gpu.c
@@ -1522,7 +1522,6 @@ void virtio_gpu_device_realize(DeviceState *qdev, Error 
**errp)
      g->ctrl_bh = virtio_bh_new_guarded(qdev, virtio_gpu_ctrl_bh, g);
      g->cursor_bh = virtio_bh_new_guarded(qdev, virtio_gpu_cursor_bh, g);
      g->reset_bh = qemu_bh_new(virtio_gpu_reset_bh, g);
-    qemu_cond_init(&g->reset_cond);
      QTAILQ_INIT(&g->reslist);
      QTAILQ_INIT(&g->cmdq);
      QTAILQ_INIT(&g->fenceq);
@@ -1535,7 +1534,6 @@ static void virtio_gpu_device_unrealize(DeviceState *qdev)
      g_clear_pointer(&g->ctrl_bh, qemu_bh_delete);
      g_clear_pointer(&g->cursor_bh, qemu_bh_delete);
      g_clear_pointer(&g->reset_bh, qemu_bh_delete);
-    qemu_cond_destroy(&g->reset_cond);
      virtio_gpu_base_device_unrealize(qdev);
  }
  
@@ -1565,9 +1563,6 @@ static void virtio_gpu_reset_bh(void *opaque)
      for (i = 0; i < g->parent_obj.conf.max_outputs; i++) {
          dpy_gfx_replace_surface(g->parent_obj.scanout[i].con, NULL);
      }
-
-    g->reset_finished = true;
-    qemu_cond_signal(&g->reset_cond);
  }
  
  void virtio_gpu_reset(VirtIODevice *vdev)
@@ -1576,11 +1571,7 @@ void virtio_gpu_reset(VirtIODevice *vdev)
      struct virtio_gpu_ctrl_command *cmd;
  
      if (qemu_in_vcpu_thread()) {
-        g->reset_finished = false;
          qemu_bh_schedule(g->reset_bh);
-        while (!g->reset_finished) {
-            qemu_cond_wait_bql(&g->reset_cond);
-        }
      } else {
          aio_bh_call(g->reset_bh);
      }

---
base-commit: 36076d24f04ea9dc3357c0fbe7bb14917375819c
change-id: 20251029-gpu-c3f45747f7ba

Best regards,